Bicritical features of the metal-insulator transition in bandwidth-controlled manganites: Single crystals of Pr_1-x(Ca_1-ySr_y)_xMnO₃

The phase change from a charge- and orbital-ordered insulator to a ferromagnetic metal has been investigated for single crystals of Pr_0.55(Ca_1-ySr_y)_0.45MnO₃ with controlled one-electron bandwidth. At 0<~y<~0.2, the ground state is the charge- and orbital-ordered insulator, while for y>~0.25 it changes to a ferromagnetic metal. At around y=0.25, the critical temperatures for the charge-orbital ordering and the ferromagnetic transition coincide with each other (≈200 K), forming the bicritical point in the electronic phase diagram. Around this insulator-metal phase boundary, prototypical colossal magnetoresistance emerges in the metallic regime, while the charge-orbital ordering is melted by a relatively low magnetic field of a few tesla in the charge- and orbital-ordered insulating regime. The phase diagrams of Pr_1-x(Ca_1-ySr_y)_xMnO₃ (0<~y<~1) with various doping levels of x=0.35, 0.45, and 0.5 indicate that such a competition between the charge-orbital ordering and the ferromagnetic metal is critically dependent on the doping level. In particular, the commensurate doping level (x=0.5) tends to stabilize the charge- and orbital-ordered state even beyond the bicritical point in the lower-temperature side of the ferromagnetic phase.